JP3552135B2 - Waveguide and wavelength conversion element using the same - Google Patents

Description

【００１３】
大きい非線形定数を有するＫＮ結晶をエピタキシャル膜導波路のＳＨＧ素子として利用する場合、ＫＮの（０１０）面すなわち（ａ−ｃ）面で成長させてその非線形定数ｄ_３２を利用できる。この場合、図１に示すように、ａ軸方向へ伝搬するａ−ｂ面内で振動する基本波からａ−ｃ面内で振動する第２高調波が得られる。表１にＫＮの非線形定数を示す。
【００１４】
【表１】
（ｐｍ／Ｖ）（基本波波長λ＝１．０６μｍ）
ｄ_３３＝−２７．４±０．３
ｄ_３２＝−１８．３±０．３
ｄ_３２＝−２０．５±０．３（λ＝８６０ｎｍ）
ｄ_３１＝−１５．８±０．３
ｄ_２４＝ １７．１±０．４
ｄ_１５＝ １６．５±０．４
しかし、ＭＯＣＶＤに限らず基板上に結晶膜をエピタキシャル成長させる場合、一般に基板と膜との格子定数が、ある程度整合していないと、基板に配向した良好なエピタキシャル膜は得にくく、ＫＮの場合でも、（ａ−ｃ）面成長を達成することは難しい。
【００１５】
ＳＴＯ結晶は立方晶系であり、格子定数は３．９０５１オングストロームである。一方、ＫＴ結晶も立方晶系であり、格子定数は３．９８９０５オングストロームとＳＴＯとかなり近いものである。
従って、斜方晶系のＫＮ結晶は、このＳＴＯ結晶又はＫＴ結晶上に成長させる場合、ＫＮのｂ，ｃ軸の格子定数と異なるため、例えば、ＳＴＯ基板の（１００）面上に良好な結晶成長は困難である。
【００１６】
ところが、ＳＴＯの（１１０）面は、図２に示すようにその格子定数がａ軸の√２倍の５．５２２６オングストロームであり、ＫＮのｂ，ｃ軸の格子定数に近くなる。
よって、ＳＴＯの（１１０）面上に、ＫＮの（ａ−ｂ）面又は（ａ−ｃ）面が成長可能となる。同様に、ＫＴの（１１０）面も５．６４１３オングストロームとなりＫＮの成長が可能となる。
【００１７】
ＫＮの導波路を作製するために、基板結晶にＳＴＯを用い、その表面にクラッド層となるＫＴ又はＫＴＮを成長させた後に、ＫＮをＭＯＣＶＤにより成長させ、導波層とした。ＳＴＯの（１１０）面には非常に良好なＫＴ、ＫＴＮ、ＫＮ結晶薄膜が成長する条件を見いだすことによって、良好なＫＮ導波路を作製することが可能となった。
【００１８】
ＳＴＯを基板にした場合、ＳＴＯの屈折率の方が導波層となるＫＮの屈折率よりも高く、このままでは、導波路とはならない。このために、ＳＴＯ基板上に、まず、ＫＮよりも若干屈折率の低いＫＴ又はＫＴＮをクラッド層として形成し、この上にＫＮを形成することで、これを導波路として利用可能とした。基板がスピネルの固溶体又はＭｇＯの場合には、屈折率は、ＫＮよりも低いために、ＫＮ膜を導波路として利用可能である。しかし、この場合は、コアとクラッドの屈折率段差が０．５程度あり、シングルモード導波路として用いるためには、ＫＮ膜の厚さは１μｍ以下と極めて薄く形成する必要が生じ、薄いＫＮ膜ではＳＨＧ素子の基本波を結合させ難い。このような場合にも、基板上にＫＴ又はＫＴＮをクラッド層として形成しておくことにより、コア及びクラッド間の屈折率段差を０．０１以下にすることができ、シングルモード導波路とする際のコア膜厚を１μｍ以上とすることができ、ＳＨＧ素子とする際の基本波の結合が容易となる。このような効果は、基板をＳＴＯとする際にも、同様に発現する。
【００１９】
次に、ＳＴＯを基板として、良好なＫＮ結晶膜を成長させる実験結果について述べる。すなわち、（１１０）面を主面とするＳＴＯ基板をＭＯＣＶＤ装置の反応室に装填し、これを設定温度まで昇温して反応室内部を設定気圧まで減圧し、出発原料として、ジピバロイルメタナトカリウム［Ｋ（Ｃ_１１Ｈ_１９Ｏ_２）］（以下、Ｋ（ＤＰＭ）という）と、ペンタエトキシニオブ［Ｎｂ（ＯＣ_２Ｈ_５）_５］（以下、ペンタエトキシニオブという）と、を装置の気化器のそれぞれに装填して、これら出発原料をそれぞれ設定温度に保つことにより昇華又は気化させ有機金属化合物ガスとし、これをそれぞれ流量制御されたＡｒキャリアガスを用いて加熱基板が配置された反応室へ導き、層流として流し、基板上にエピタキシャル層を析出させた。また、出発原料からの各酸化物の生成には酸化反応をともなうため、反応ガスに一定量の酸素を添加してもよい。原料ガスをＳＴＯ基板（１１０）面上に流すことによりＫＮエピタキシャル単結晶薄膜が成長できた。
【００２０】
図３に示すように、ＳＴＯ（１１０）面の基板１上に基板より屈折率の大きいＫＮエピタキシャル層２を形成した。
エックス線回折実験によると、ＳＴＯ（１１０）面上にＫＮ膜の（ａ−ｃ）面が成長されていることが確認された。図４に示すようにエックス線回折により、ＳＴＯ基板（１１０）面上のＫＮ膜がＳＴＯにかなり近い回折パターンを有していることから明らかとなった。図中、縦軸は回折ビーム強度を、横軸は回折ビーム角度を示す。図から明らかなように、回折ビーム角度２θにおいて、３１度及び３２度付近にＫＮ結晶面（０２０）及びＳＴＯ結晶面（１１０）のピークが、６６度及び６８度付近にＫＮ結晶面（０４０）及びＳＴＯ結晶面（２２０）のピークが近接して表れている。また、同様に作成したＫＴエピタキシャル導波路のスラブ型波長変換素子についてもＫＴ膜がＳＴＯにかなり近い回折パターンを有していた。
【００２１】
先に述べたように、このままではＫＮ膜を導波路として利用できないため、次に、ＳＴＯ基板及びＫＮエピタキシャル膜導波路の間に、ＫＮより屈折率の小さいＫＴａ_ｘＮｂ_１−ｘＯ_３（０＜ｘ≦１）、例えば、ＫＴ結晶からなるクラッド層を形成した波長変換素子を作製した。
このＳＴＯ基板、ＫＴ結晶クラッド及びＫＮ結晶導波路からなるＳＨＧ素子を作製するために、上記ＭＯＣＶＤ装置により、ＳＴＯ基板結晶の（１１０）面上に、まずＫＴを成長させ、続いて、ＫＮを同装置内で成長させ、導波層とした。この様に、ＳＴＯの（１１０）面に非常に良好なＫＴ及びＫＮ結晶薄膜が成長する条件を見いだすことによって、良好なＫＮ導波路を作製することが可能である。
【００２２】
すなわち、ＭＯＣＶＤ装置の反応室にＳＴＯ基板を装填し、温度、気圧を設定し、出発原料として、Ｋ（ＤＰＭ）とペンタエトキシタンタル［Ｔａ（ＯＣ_２Ｈ_５）_５］（以下、ペンタエトキシタンタルという）と、をガス化し、これらをそれぞれキャリアガスでＳＴＯ基板上に流し、基板上で反応させることによりＫＴエピタキシャル単結晶薄膜が成長できた。なお、ＫＴａ_ｘＮｂ_１−ｘＯ_３（０＜ｘ≦１）エピタキシャル単結晶薄膜を形成するときは出発原料として、Ｋ（ＤＰＭ）、ペンタエトキシニオブ及びペンタエトキシタンタルを用い、それぞれ原料ガス流量制御によってエピタキシャル層成分の原子比を変えて成膜する。原子比を変えることによりクラッド層及び導波路の屈折率差を制御することができる。
【００２３】
図５に示すような、ＳＴＯ（１１０）面の基板１上にＫＴエピタキシャルクラッド層３及びこれより屈折率の大きいＫＮエピタキシャル層２が順に形成されたスラブ型導波路を作成した。以上の実施例では、ＳＴＯ（１１０）を基板として用いた場合について述べたが、格子定数が比較的ＫＮと近く、立方晶系であるスピネル［（ＭｇＯ）（Ａｌ_２Ｏ_３）］の固溶体又はＭｇＯの（１１０）面を基板としても同様な構造の良好なスラブ型導波路を得ることができた。
【００２４】
こうして作製した２次元導波路のＫＮエピタキシャル層２上に、図６に示すように、ＳｉＯ_２のストライプ４を形成し、さらに、基本波と第２高調波との位相整合条件を満足させ安定にこれを保つために、導波路の近傍にストライプ電極５を平行して形成し、３次元導波路即ちチャネル型導波路を作製した。その電極間に電界を印加し、１次の電気光学効果によって、基本波モード及び第２高調波モードの実効屈折率を、共に電場で変化させて位相整合を得る。またＳｉＯ_２ストライプ４上に半導体レーザ制御用回折格子６を形成してもよい。
【００２５】
基本波λ＝８６０ｎｍをこの屈折率の大きいエピタキシャル層２に導波させることにより、良好な光閉じ込めが達成され、第２高調波λ＝４３０ｎｍが高変換効率で得られた。さらに、かかるＳＨＧ素子では、ＫＮはＬＮ、ＬＴ、ＬＮＴに比較して、光損傷しきい値が高いので、高い出力まで安定した動作が得られた。なお、ＫＴエピタキシャルクラッド層の膜厚は基本波の波長以上に設定した。エバネッセント波のしみ出しがＳＴＯ基板に及ぶと伝搬損失が大きくなるので好ましくないからである。ＫＮエピタキシャル導波路層の膜厚は１〜５μｍに設定した。ＫＴ層との実効屈折率差の関係で単一モードの導波が可能であり、また１μｍ未満だと光結合が難しくなり、５μｍ超だと多モードとなり光パワー密度が低下するからである。
【００２６】
また、ＳｉＯ_２ストライプ４下に形成されるＫＮチャネル型導波路両側のＫＮ層上に、バッファ層を設けて平行電極５を形成してもよく、チャネル導波路では、上記誘電体装荷型の他に、スラブＫＮ導波路層をエッチングによりリッジ型導波路を形成することもできる。また、金属装荷型、電圧誘起型もチャネル型導波路に用い得る。
【００２７】
更にまた、本発明によれば、上記素子の３次元導波路に交差する周期的分極反転層を形成して、非線形光学効果による第２高調波出力がその伝播に伴ってコヒーレンス長毎に極大極小を周期的に繰返すことを利用して、コヒーレンス長毎に発生する分極の符号を交互に反転させて、第２高調波の出力の加算により出力を増大させる疑似位相整合（ＱＰＭ）の波長変換素子を形成することもできる。さらに導波路の膜厚、オーバーレイヤなどの種々の位相整合方法も用いることもできる。
【００２８】
【発明の効果】
本発明によれば、ＭＯＣＶＤ法を用い、ＳｒＴｉＯ_３、スピネル固溶体、ＭｇＯのいずれかの結晶基板とこの上に形成されたＫＴ又はＫＴＮクラッド層と高光損傷耐性のＫＮなどの強誘電体結晶導波路、及びこれを有する波長変換素子が得られる。さらに、キャリアーガスの流量を変えることにより、容易に、結晶組成を変えることができるので、導波路の屈折率、膜厚を精密に制御することができ製造容易な波長変換素子が得られる。
【図面の簡単な説明】
【図１】ＫＮｂＯ_３の結晶格子を示す概略斜視図である。
【図２】ＳｒＴｉＯ_３の結晶格子を示す概略斜視図である。
【図３】本発明による実施例の波長変換素子の概略断面図である。
【図４】エックス線回折により、本発明による実施例のＳＴＯ基板（１１０）面上のＫＮ膜の回折パターンを示す回折ビーム強度−回折角度の特性を示すグラフである。
【図５】本発明による他の実施例の波長変換素子の概略断面図である。
【図６】本発明による他の実施例の波長変換素子の概略斜視図である。
【主要部分の符号の説明】
１ ＳＴＯ（１１０）面基板
２ ＫＮエピタキシャル層
３ ＫＴエピタキシャルクラッド層
４ ＳｉＯ_２ストライプ
５ ストライプ電極
６ 半導体レーザ制御用回折格子[0001]
[Technical field to which the present invention pertains]
The present invention relates to a nonlinear optical crystal waveguide utilizing second harmonic generation , and a wavelength conversion element having the same , a so-called SHG element.
[0002]
[Prior art]
In the wavelength conversion element, it is necessary to satisfy a phase matching condition in order to efficiently generate the second harmonic. The phase matching method uses the birefringence of the substrate crystal of the wavelength conversion element to satisfy the phase matching condition using angle tuning, temperature tuning, and electric field tuning. Various methods such as film thickness, overlayer, Cerenkov radiation, and quasi-phase matching have been attempted.
[0003]
LiTaO ₃ (hereinafter referred to as LT) and LiNbO ₃ (hereinafter referred to as LN) composed of lithium Li, potassium K, tantalum Ta, niobium Nb, oxygen O, phosphorus P, titanium Ti, and the like are provided on the substrate of the wavelength conversion element. , KTiOPO ₄ (hereinafter, referred to as KTP) or the like is preferably used as a ferroelectric crystal having a large nonlinear optical constant.
[0004]
Conventionally, in the wavelength conversion element, (1) a waveguide is manufactured by Ti diffusion on an LN substrate, and the phase velocities of a fundamental wave and a harmonic wave propagating through the waveguide are matched with adjusting the temperature of the element. That is, phase matching is achieved by satisfying the phase matching conditions and performing wavelength conversion, or (2) by forming an LN crystal film on the LT substrate by a liquid tank epitaxial method and strictly controlling the thickness of the waveguide. And (3) a quasi-phase matching wavelength conversion element that forms a waveguide and a domain-inverted grating orthogonal to the waveguide on an LN, LT, or KTP substrate to achieve phase matching. I have.
[0005]
In order to obtain a large conversion efficiency, it is necessary to use a large non-linear constant, the (1), the nonlinear constants utilized in the method of (2) is d _31, this value is 5小 さ い 7 pm / V, which is small. In the method (3), a large nonlinear constant d ₃₃ can be used. However, in the quasi-phase matching method, an effective nonlinear constant d _eff = (2 / π) d ₃₃ is obtained. Disadvantageous. When LN is used, the effective nonlinear constant can be increased, but there is a disadvantage that the optical damage resistance to the blue light obtained by the wavelength conversion is small and a large blue light output cannot be obtained.
[0006]
Therefore, an attempt has been made to use KNbO ₃ (hereinafter, referred to as KN), which is a nonlinear optical crystal having a large nonlinear constant and a large optical damage resistance, as a substrate.
[0007]
[Problems to be solved by the invention]
In forming the KN waveguide, a KN thin film is formed by a method of performing ion exchange on a KN crystal substrate, a method of performing ion implantation on the substrate, or a sputtering method on a substrate of magnesium oxide MgO or spinel MgOAl ₂ O _3. Although there have been attempts to fabricate waveguides by forming them, it has been difficult to produce high-quality KN waveguides, and no practical wavelength conversion element has been obtained.
[0008]
Accordingly, an object of the present invention is to provide a waveguide of a ferroelectric crystal having high optical damage resistance formed on a crystal substrate by a thin film crystal forming means such as a metal organic chemical vapor deposition method (hereinafter referred to as MOCVD). It is to provide a wavelength conversion element having the following.
[0009]
[Means for Solving the Problems]
The waveguide of the present invention is formed on a substrate made of any one of SrTiO ₃ (hereinafter, referred to as STO) of titanium oxide containing strontium Sr, a solid solution of spinel [(MgO) (Al ₂ O ₃ )] , and MgO. A ferroelectric material such as KN formed on a clad layer such as formed KTaO ₃ (hereinafter referred to as KT) or KTa _x Nb _1-x O ₃ ( 0 <x <1) (hereinafter referred to as KTN). It is characterized by being made of crystals.
In addition, the wavelength conversion element of the present invention includes a substrate made of any one of STO of titanium oxide containing strontium Sr, a solid solution of spinel [(MgO) (Al ₂ O ₃ )], and a crystal of MgO. It is characterized by having a formed cladding layer such as KT and KTN and a waveguide made of a ferroelectric crystal such as KN.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment according to the present invention will be described with reference to the drawings.
The inventor of the present invention epitaxially grows KT or KTN as a cladding layer on an STO substrate by MOCVD and further forms a KN epitaxial layer as a core layer thereon, and this epitaxial layer becomes a waveguide with extremely few lattice defects. And arrived at the wavelength conversion element of the present invention.
[0011]
The KN crystal of the ferroelectric crystal having high photodamage resistance is an orthorhombic system which is a rectangular parallelepiped lattice having three different sides in the coordinates shown in FIG. 1, and has a lattice constant of a = 3.9692 Å and b = 5. 0.6896 angstroms, c = 5.7256 angstroms. The relationship between the orthorhombic polarization and the nonlinear sensation tensor and electric field is given by Equation 1 (where P is a polarization component, d is a second-order nonlinear constant, E is a fundamental wave electric field component, and a, b, and c are axial directions). Is shown).
[0012]
(Equation 1)

[0013]
When utilizing KN crystal having a large nonlinear coefficient as SHG element epitaxial Makushirube waveguide, and can use their nonlinear constant _{d 32} grown in (010) plane or (a-c) plane of KN. In this case, as shown in FIG. 1, a second harmonic wave oscillating in the ac plane is obtained from a fundamental wave oscillating in the ab plane propagating in the a-axis direction. Table 1 shows the nonlinear constants of KN.
[0014]
[Table 1]
(Pm / V) (fundamental wave wavelength λ = 1.06 μm)
d ₃₃ = −27.4 ± 0.3
d ₃₂ = −18.3 ± 0.3
d ₃₂ = -20.5 ± 0.3 (λ = 860 nm)
d ₃₁ = −15.8 ± 0.3
d ₂₄ = 17.1 ± 0.4
d ₁₅ = 16.5 ± 0.4
However, when a crystal film is epitaxially grown on a substrate without being limited to MOCVD, it is generally difficult to obtain a good epitaxial film oriented on the substrate unless the lattice constant between the substrate and the film is matched to some extent. (Ac) It is difficult to achieve plane growth.
[0015]
The STO crystal is cubic and has a lattice constant of 3.9051 Å. On the other hand, the KT crystal is also cubic, and has a lattice constant of 3.98905 angstroms, which is quite close to that of STO.
Therefore, when the orthorhombic KN crystal is grown on this STO crystal or KT crystal, it differs from the lattice constant of the bN and c axes of KN, and for example, a good crystal is formed on the (100) plane of the STO substrate. Growth is difficult.
[0016]
However, as shown in FIG. 2, the (110) plane of the STO has a lattice constant of 5.5226 Å, which is √2 times the a-axis, and is close to the lattice constants of the KN on the b and c axes.
Therefore, the (ab) plane or the (ac) plane of KN can be grown on the (110) plane of the STO. Similarly, the (110) plane of KT is also 5.6413 Å, and KN can be grown.
[0017]
In order to manufacture a waveguide of KN, STO was used as a substrate crystal, KT or KTN to be a clad layer was grown on the surface thereof, and then KN was grown by MOCVD to form a waveguide layer. By finding the conditions under which very good KT, KTN, and KN crystal thin films grow on the (110) plane of the STO, a good KN waveguide can be manufactured.
[0018]
When the STO is used as a substrate, the refractive index of the STO is higher than the refractive index of KN to be the waveguide layer, and the waveguide does not form as it is. For this purpose, first, KT or KTN having a slightly lower refractive index than KN is formed as a cladding layer on the STO substrate, and KN is formed thereon, so that it can be used as a waveguide. When the substrate is a solid solution of spinel or MgO, the refractive index is lower than that of KN, so that the KN film can be used as a waveguide. However, in this case, the refractive index step between the core and the clad is about 0.5, and the KN film needs to be formed as extremely thin as 1 μm or less to be used as a single mode waveguide. Then, it is difficult to couple the fundamental wave of the SHG element. Even in such a case, by forming KT or KTN as a cladding layer on the substrate, the refractive index step between the core and the cladding can be reduced to 0.01 or less. Can have a core thickness of 1 μm or more, which facilitates the coupling of fundamental waves when forming an SHG element. Such an effect is similarly exhibited when the substrate is STO.
[0019]
Next, experimental results for growing a good KN crystal film using STO as a substrate will be described. That is, an STO substrate having a (110) plane as a main surface is loaded into a reaction chamber of an MOCVD apparatus, the temperature is raised to a set temperature, and the inside of the reaction chamber is depressurized to a set pressure. Potassium tanato [K (C ₁₁ H ₁₉ O ₂ )] (hereinafter, referred to as K (DPM)) and pentaethoxy niobium [Nb (OC ₂ H ₅ ) ₅ ] (hereinafter, referred to as pentaethoxy niobium) of the apparatus. Each of the starting materials is charged into each of the vaporizers, and the starting materials are sublimated or vaporized by keeping them at the set temperatures to form organometallic compound gases. It was led into a chamber, flowed as a laminar flow, and an epitaxial layer was deposited on the substrate. In addition, a certain amount of oxygen may be added to the reaction gas since the generation of each oxide from the starting material involves an oxidation reaction. The KN epitaxial single crystal thin film could be grown by flowing the source gas on the STO substrate (110).
[0020]
As shown in FIG. 3, a KN epitaxial layer 2 having a higher refractive index than the substrate was formed on the substrate 1 on the STO (110) plane.
According to the X-ray diffraction experiment, it was confirmed that the (ac) plane of the KN film was grown on the STO (110) plane. As shown in FIG. 4, X-ray diffraction revealed that the KN film on the (110) surface of the STO substrate had a diffraction pattern quite similar to that of the STO. In the figure, the vertical axis indicates the diffraction beam intensity, and the horizontal axis indicates the diffraction beam angle. As is clear from the figure, at the diffraction beam angle 2θ, the peaks of the KN crystal plane (020) and the STO crystal plane (110) near 31 and 32 degrees, and the KN crystal plane (040) near 66 and 68 degrees. And the peaks of the STO crystal plane (220) appear close to each other. Also, in the slab-type wavelength conversion element of the KT epitaxial waveguide produced similarly, the KT film had a diffraction pattern quite similar to STO.
[0021]
As described above, since the KN film cannot be used as a waveguide as it is, next, between the STO substrate and the KN epitaxial film waveguide, KTa _x Nb _1-x O ₃ (0 <X ≦ 1) For example, a wavelength conversion element having a clad layer made of a KT crystal was manufactured.
In order to manufacture an SHG element composed of the STO substrate, the KT crystal clad, and the KN crystal waveguide, KT is first grown on the (110) plane of the STO substrate crystal by the MOCVD apparatus, and then the KN is grown. The waveguide layer was grown in the device. As described above, it is possible to manufacture a good KN waveguide by finding the conditions under which very good KT and KN crystal thin films grow on the (110) plane of the STO.
[0022]
That is, loading the STO substrate in a reaction chamber of the MOCVD apparatus, the temperature, set the pressure, as a starting material, K (DPM) and pentaethoxytantalum _{[Ta (OC 2 H 5)} 5] ( hereinafter, referred pentaethoxytantalum And) were gasified, each of which was flowed on an STO substrate with a carrier gas, and reacted on the substrate to grow a KT epitaxial single crystal thin film. When a KTa _x Nb _1-x O ₃ (0 <x ≦ 1) epitaxial single crystal thin film is formed, K (DPM), pentaethoxyniobium and pentaethoxytantalum are used as starting materials, and the respective material gas flow rates are controlled. The film is formed by changing the atomic ratio of the components of the epitaxial layer. The refractive index difference between the cladding layer and the waveguide can be controlled by changing the atomic ratio.
[0023]
As shown in FIG. 5, a slab type waveguide in which a KT epitaxial cladding layer 3 and a KN epitaxial layer 2 having a higher refractive index than the KT epitaxial cladding layer 3 were formed on a STO (110) plane substrate 1 was prepared. In the above embodiment, the case where STO (110) is used as the substrate has been described. However, the lattice constant is relatively close to KN, and the solid solution of spinel [(MgO) (Al ₂ O ₃ )], which is a cubic system, A good slab waveguide having a similar structure could be obtained even when the (110) plane of MgO was used as a substrate.
[0024]
As shown in FIG. 6, a stripe 4 of SiO ₂ is formed on the KN epitaxial layer 2 of the two-dimensional waveguide thus manufactured, and furthermore, the phase matching condition between the fundamental wave and the second harmonic is satisfied to stably. In order to maintain this, a stripe electrode 5 was formed in parallel near the waveguide, and a three-dimensional waveguide, that is, a channel waveguide was manufactured. An electric field is applied between the electrodes, and both the effective refractive indexes of the fundamental mode and the second harmonic mode are changed by the electric field by the first-order electro-optic effect to obtain phase matching. Further, the semiconductor laser control diffraction grating 6 may be formed on the SiO ₂ stripe 4.
[0025]
By guiding the fundamental wave λ = 860 nm to the epitaxial layer 2 having a large refractive index, good optical confinement was achieved, and the second harmonic λ = 430 nm was obtained with high conversion efficiency. Further, in the SHG element, since KN has a higher optical damage threshold value than LN, LT, and LNT, stable operation up to a high output was obtained. The thickness of the KT epitaxial cladding layer was set to be equal to or longer than the wavelength of the fundamental wave. This is because if the evanescent wave seeps into the STO substrate, the propagation loss increases, which is not preferable. The thickness of the KN epitaxial waveguide layer was set to 1 to 5 μm. This is because single-mode waveguide is possible due to the effective refractive index difference from the KT layer, and if it is less than 1 μm, optical coupling becomes difficult, and if it exceeds 5 μm, it becomes multimode and the optical power density decreases.
[0026]
Further, a buffer layer may be provided on the KN layer on both sides of the KN channel type waveguide formed below the SiO ₂ stripe 4 to form the parallel electrode 5. Alternatively, a ridge type waveguide can be formed by etching the slab KN waveguide layer. Further, a metal loaded type and a voltage induced type can be used for the channel type waveguide.
[0027]
Further, according to the present invention, a periodic domain-inverted layer intersecting the three-dimensional waveguide of the device is formed, and the second harmonic output due to the nonlinear optical effect is maximized and minimized for each coherence length with its propagation. The wavelength conversion element of a quasi phase matching (QPM) in which the sign of the polarization generated for each coherence length is alternately inverted by using the cyclic repetition of the above, and the output is increased by adding the output of the second harmonic. Can also be formed. Further, various phase matching methods such as a waveguide film thickness and an overlayer can also be used.
[0028]
【The invention's effect】
According to the present invention, a crystal substrate of SrTiO ₃ , spinel solid solution, or MgO, a KT or KTN cladding layer formed thereon, and a ferroelectric crystal waveguide such as KN having high optical damage resistance are formed by MOCVD. , And a wavelength conversion element having the same. Further, since the crystal composition can be easily changed by changing the flow rate of the carrier gas, the refractive index and the film thickness of the waveguide can be precisely controlled, and a wavelength conversion element which is easy to manufacture can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic perspective view showing a crystal lattice of KNbO ₃ .
FIG. 2 is a schematic perspective view showing a crystal lattice of SrTiO ₃ .
FIG. 3 is a schematic sectional view of a wavelength conversion element according to an embodiment of the present invention.
FIG. 4 is a graph showing a diffraction beam intensity-diffraction angle characteristic showing a diffraction pattern of a KN film on a surface of an STO substrate (110) of an example according to the present invention by X-ray diffraction.
FIG. 5 is a schematic sectional view of a wavelength conversion element according to another embodiment of the present invention.
FIG. 6 is a schematic perspective view of a wavelength conversion element according to another embodiment of the present invention.
[Description of Signs of Main Parts]
Reference Signs List 1 STO (110) plane substrate 2 KN epitaxial layer 3 KT epitaxial cladding layer 4 SiO ₂ stripe 5 Stripe electrode 6 Diffraction grating for semiconductor laser control

Claims (13)

A waveguide formed on a cladding layer provided on a substrate,
Wherein the substrate is SrTiO _3, Ri Do spinel _{[(MgO) (Al 2 O} 3)] of either solid solutions and MgO crystals,
A waveguide, wherein the cladding layer is made of a crystal of KTa _x Nb _1-x O ₃ (0 ≦ x ≦ 1) . 2. The waveguide according to claim 1, wherein the waveguide is made of a ferroelectric crystal. _3. The waveguide according to claim 1, wherein said waveguide is made of KNbO ₃ crystal. The cladding layer is said SrTiO _3, spinel _{[(MgO) (Al 2 O} 3)] of the solid solution and one of the crystals of MgO (110) guiding according to claim 3, characterized in that formed on the surface Wave path. The cladding layer is made of KTaO _Three 5. The waveguide according to claim 4, wherein the waveguide is formed by: The waveguide according to any one of claims 1 to 5, wherein the waveguide is a channel waveguide. The waveguide according to any one of claims 1 to 5, wherein the waveguide is a slab waveguide. SrTiO _Three , Spinel [(MgO) (Al _Two O _Three A) a substrate comprising a solid solution of any of the above and a crystal of MgO, and KTa formed on the substrate _x Nb _1-x O _Three A wavelength conversion element comprising: a cladding layer made of a crystal (0 ≦ x ≦ 1); and a waveguide made of a ferroelectric crystal formed on the cladding layer. The waveguide is KNbO _Three 9. The wavelength conversion element according to claim 8, comprising a crystal of: The clad layer is made of the SrTiO _Three , Spinel [(MgO) (Al _Two O _Three )] Of any of the solid solution and the MgO crystal (110) The wavelength conversion element according to claim 9, wherein the wavelength conversion element is formed on a surface. The cladding layer is made of KTaO _Three 9. The wavelength conversion element according to claim 8, wherein the wavelength conversion element is formed by: The wavelength conversion element according to claim 8, wherein the waveguide is a channel waveguide. The wavelength conversion element according to claim 8, wherein the waveguide is a slab waveguide.

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